This invention relates to a spread-spectrum demodulator, more particularly to a demodulator for code-division multiple-access (CDMA) communications.
CDMA is employed in personal communication systems and other mobile communication systems in which M stations transmit simultaneously over the same frequency band. (M is an integer greater than one.) Each station uses a different pseudo-random noise signal as a spreading code, chopping each bit symbol of transmit data into smaller units, referred to as "chips," with pseudo-random values. A receiving station receives a signal equal to the sum of the M signals transmitted by the transmitting stations, and demodulates this received signal to M separate signals by, for example, multiplying the received signal by the M different spreading codes.
If the spreading codes are mutually orthogonal over each symbol duration and the mobile stations transmit in synchronization, the demodulated signals will be free of interference. In practice, however, the codes are not perfectly orthogonal, synchronization is lacking, and the signal transmitted by each mobile station interferes somewhat with the signals transmitted by other stations. As the number of stations increases, so does the amount of interference, until the desired signal from each station is swamped by noise. This interference, referred to as co-channel interference, becomes the factor limiting the number of stations that can access the system simultaneously.